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  • Chapter 10

    Nuclear PhysicsChapter 10: Nuclear Physics

    Homework: All questions on the Multiple-Choice and the odd-numbered questions onExercises sections at the end of the chapter.

  • Copyright Houghton Mifflin Company. All rights reserved. 10 | 2

    Nuclear Physics

    The characteristics of the atomic nucleus areimportant to our modern society.

    Diagnosis and treatment of cancer and otherdiseases

    Geological and archeological dating

    Chemical analysis

    Nuclear energy and nuclear diposal

    Formation of new elements

    Radiation of solar energy

    Household smoke detector

    Intro

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    Early Thoughts about Elements

    The Greek philosophers (600 200B.C.) were the first people to speculateabout the basic substances of matter.

    Aristotle speculated that all matter onearth is composed of only fourelements: earth, air, fire, and water.

    He was wrong on all counts!

    Section 10.1

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    Symbols of the Elements

    Swedishchemist, JonsJakob Berzelius(early 1800s)used one or twoletters of theLatin name todesignate eachelement.

    Section 10.1

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    Symbols of the Elements

    Since Berzelius time most elementshave been symbolized by the first oneor two letters of the English name.

    YOU are expected to know the namesand symbols of the 45 elements listedon Table 10.2.

    Section 10.1

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    Names and Symbols of Common Elements

    Section 10.1

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    The Atom

    All matter is composed of atoms.

    An atom is composed of three subatomicparticles: electrons (-), protons (+), andneutrons (0)

    The nucleus of the atom contains the protonsand the neutrons (also called nucleons.)

    The electrons surround (orbit) the nucleus.

    Electrons and protons have equal butopposite charges.

    Section 10.2

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    Major Constituents of an Atom

    Section 10.2

    U=unified atomic mass units

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    The Atomic Nucleus

    Protons and neutrons have nearly thesame mass and are 2000 times moremassive than an electron.

    Discovery Electron (J.J. Thomson in1897), Proton (Ernest Rutherford in1918), and Neutron (James Chadwick in1932)

    Section 10.2

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    Rutherford's Alpha-Scattering Experiment

    J.J. Thomsons plum pudding model predicted thealpha particles would pass through the evenlydistributed positive charges in the gold atoms.

    a particle = helium nucleus

    Section 10.2

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    Rutherford's Alpha-Scattering Experiment

    Only 1 out of 20,000 alpha particlesbounced back.

    Rutherford could only explain this byassuming that each gold atom had itspositive charge concentrated in a verysmall nucleus.

    Diameter of nucleus = about 10-14 m

    Electron orbit diameter = about 10-10 m

    Atomic Mass is concentrated in thenucleus (>99.97%)

    Section 10.2

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    Atomic Mass is Concentrated in theNucleus!

    Therefore the volume (or size) of anatom is determined by the orbitingelectrons. The diameter of an atom is approximately

    10,000 times the diameter of the nucleus.

    If only nuclear material (protons andneutrons) could be closely packed into asphere the size of a ping-pong ball itwould have the incredible mass of 2.5billion metric tons!

    Section 10.2

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    VisualRepresentation ofa Nucleus

    Tightly PackedProtons andNeutrons

    Section 10.2

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    Atomic Designations

    Atomic Number (Z) the # of protons in thenucleus (defines the element the # ofprotons is always the same for a givenelement)

    Atomic Number also designates the numberof electrons in an element.

    If an element either gains or loses electrons,the resulting particle is called and ion.

    For example, if a sodium atom (Na) loses anelectron it becomes a sodium ion (Na+.)

    Section 10.2

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    More Atomic Designations

    Mass Number (A) protons + neutrons, orthe total number of nucleons

    Isotope when the number of neutrons varyin the nucleus of a given element (alwayssame number of protons)

    Only 112 elements are known, but the totalnumber of isotopes is about 2000.

    Section 10.2

  • Copyright Houghton Mifflin Company. All rights reserved. 10 | 16Section 10.2

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    Isotopes

    Some elements have several isotopes(like carbon 12C, 13C, 14C)

    Isotopes of a single element have thesame chemical properties (due to samenumber of electrons), but they havedifferent masses (due to varying numberof neutrons.)

    Due to their various masses isotopesbehave slightly different during reactions.

    Section 10.2

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    Carbon Isotopes - Example

    SymbolProtons

    (Z)Neutrons

    (N)Mass #

    (A)

    12C 6 6 12

    13C 6 7 13

    14C 6 8 14

    Section 10.2

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    Three Isotopes of Hydrogen

    In naturally occurring Hydrogen - 1 atom in 6000 isdeuterium and 1 in 10,000,000 is tritium. Heavy water = D2O

    Section 10.2

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    CommonIsotopes ofsome of theLighterElements

    Section 10.2

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    Determining the Composition of an Atom

    Atomic Number (Z) = 9

    \ protons = 9 & electrons = 9

    Mass Number (A) = 19

    A = N + Z {N = Neutron Number}

    \ N = A Z = 19 9 = 10

    neutrons = 10

    Section 10.2

    9

    Determine the number of protons, electrons,and neutrons in the fluorine atom 19F

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    Atomic Review

    Protons & Neutrons in nucleus

    Electrons orbit around nucleus

    Mass Number (A) = protons + neutrons

    Atomic Number (Z) = # of protons

    Neutron Number (N) = # of neutrons

    Isotope an element with different # ofneutrons (same # of protons)

    Section 10.2

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    Atomic Mass

    The weighted average mass of an atom ofthe element in a naturally occurring sample

    The Atomic Mass is measured in unifiedunifiedatomic mass units (u)atomic mass units (u) basically the weightof a proton or neutron.

    The 12C atom is used as the standard, and isassigned the Atomic Mass of exactlyexactly 12 u.

    The weighted average mass of all carbon isslightly higher than 12 (12.011) becausesome is 13C and 14C.

    Section 10.2

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    Schematic Drawing of a Mass Spectrometer

    Copyright Houghton Mifflin Company

    The ion with the greatest mass is deflected the least, the ionwith the least mass is deflected the most.

    Section 10.2

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    A Mass SpectrogramShowing the Three Isotopes of Neon and their relative

    abundances

    Section 10.2

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    Calculating an Elements Atomic Mass

    Naturally occurring chlorine is a mixtureconsisting of 75.77% 35Cl (atomic mass =34.97 u) and 24.23% 37Cl (atomic mass =36.97 u). Calculate the atomic mass for theelement chlorine.

    Calculate the contribution of each Cl isotope.

    0.7577 x 34.97 u = 26.50 u (35Cl)

    0.2423 x 36.97 u = 8.96 u (37Cl)

    Total = 35.46 u = Atomic Mass for Cl

    Section 10.2

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    Fundamental Forces of Nature - Review

    We have previously discussed twofundamental forces of nature gravitationaland electromagnetic.

    The electromagnetic force between a proton(+) and an electron (-) is 1039 greater thanthe gravitational forces between the twoparticles.

    Therefore the electromagnetic forces arethe only important forces on electrons andare responsible for the structure of atoms,molecules and all matter in general.

    Section 10.2

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    The (Strong) Nuclear Force

    Remember that the nucleus of any atom isextremely small and packed with acombination of neutrons and protons (+.)

    According to Coulombs Law like chargesrepel each other. Therefore the repulsiveforces in a nucleus are huge and the nucleusshould fly apart.

    There must exist a third fundamental forcethat somehow holds the nucleus together.

    For a large nucleus the forces arecomplicated.

    Section 10.2

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    Large Nucleus & the Nuclear Force

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